An intraluminal prosthesis is a medical device used in the treatment of diseased bodily lumens. One type of intraluminal prosthesis used in the repair and/or treatment of diseases in various body vessels is a stent. A stent is a generally longitudinal tubular device formed of biocompatible material which is useful to open and support various lumens in the body. For example, stents may be used in the vascular system, urogenital tract, esophageal tract, tracheal/bronchial tubes and bile duct, as well as in a variety of other applications in the body. These devices are implanted within the vessel to open and/or reinforce collapsing or partially occluded sections of the lumen.
Stents generally include an open flexible configuration. This configuration allows the stent to be inserted through curved vessels. Furthermore, this configuration allows the stent to be configured in a radially compressed state for intraluminal catheter implantation. Once properly positioned adjacent the damaged vessel, the stent is radially expanded so as to support and reinforce the vessel. Radial expansion of the stent may be accomplished by inflation of a balloon attached to the catheter or the stent may be of the self-expanding variety which will radially expand once deployed. Tubular shaped structures, which have been used as intraluminal vascular stents, have included helically wound coils which may have undulations or zig-zags therein, slotted stents, ring stents, braided stents and open mesh wire stents, to name a few. Super-elastic materials and shape memory materials have been used to form stents.
A graft, including a shunt, for example, a dialysis shunt, is another commonly known type of intraluminal prosthesis which is used to repair, replace or bridge various body vessels. A graft provides a lumen through which fluids, such as blood, may flow. Moreover, a graft is often configured as being generally impermeable to blood to inhibit substantial leakage of blood therethrough. Grafts are typically hollow tubular devices that may be formed of a variety of materials, including textile and non-textile materials.
A stent and a graft may be combined into a stent-graft endoprosthesis to combine the features and advantages of each. For example, tubular coverings have been provided on the inner and/or outer surfaces of stents to form stent-grafts. It is often desirable to use a thin-walled graft or covering in the stent-graft endoprosthesis to minimize the profile of the endoprosthesis and to maximize the flow of blood through the endoprosthesis. In such cases non-textile materials, such as polymeric tubes or sheets formed into tubes, are often used. Expanded polytetrafluoroethylene or e-PTFE is one common polymeric material used as the graft portion or covering of a stent-graft endoprosthesis.
Polytetrafluoroethylene (PTFE) is commonly used for implantable medical devices due to its chemical stability, bio-stability and bio-inertness. It is also highly hydrophobic. The mechanically stretched, expanded form (ePTFE) is microscopically porous and possesses the stability and inertness properties of PTFE. The hydrophobicity of PTFE and ePTFE and the low adsorption of proteins by PTFE and ePTFE were regarded as favourable characteristics for good performance in vascular vessels. Certain considerations, however, are present with implanted PTFE or ePTFE materials, including thrombosis and anastomosis stenosis by intima hyperplasia.
A thrombus is the formation of a solid body composed of elements of the blood, e.g., platelets, fibrin, red blood cells, and leukocytes. Thrombus formation is caused by blood coagulation and platelet adhesion to, and platelet activation on, foreign substances. When this occurs, a graft is occluded by such thrombotic material, which in turn, results in decreased patency for the graft. Accordingly, more stringent selection criteria are necessary for small caliber vascular graft materials because the small diameters of these grafts magnify the problem of deposition of such thrombotic material on the luminal surfaces of the graft.
Biologically designed PTFE or ePTFE surfaces, e.g. by heparin coating of the graft, can reduce platelet adherence and the intima proliferation. Heparin, however, has the problem of a physiological decay in biological activity and in some cases iatrogenic inactivation with protaminsulfate.
Endothelial cells as the inner lining of blood vessels are known to provide a hemocompatible surface. ePTFE as such, however, does not support endothelialization in vivo. Various attempts have been made to achieve an adherent layer of endothelial cells on the surface. Seeding the cells in a system with dynamic pressure and flow in vitro has been described as one way to obtain an endothelial lining, which resists the shear stress of physiological blood flow.
Some ways of surface modification of ePTFE, which influence polarity and surface energy, have been successfully attempted, such as surface treatment of ePTFE with energetic ions, either as plasma treatment or as ion beam irradiation of ePTFE. For example, amide and amine groups were deposited onto PTFE and ePTFE materials by radio frequency (RF) glow discharge plasma treatment of butylamine, and bovine aortic endothelial cells were then seeded on the amide/amine coated materials. See, Tseng et al., “Effects Of Amide And Amine Plasma-Treated ePTFE Vascular Grafts On Endothelial Cell Lining In An Artificial Circulatory System”, Journal Of Biomedical Materials Research, 1998, Volume 42, pages 188-189. Such RF plasma treatment is typically done at low energy levels to deposit a thin coating, such as 30 to 100 Angstrom thick coating as reported in Tseng. While, improved cell adherence and proliferation for RF plasma treated ePTFE was reported as compared to the non-treated ePTFE, long term stability of the cell layer and the long term stability of the plasma treatment were not adequate because plasma treatment usually only has a relatively short term effect as the modified molecules tend to migrate into the bulk of the polymer. Ion beam irradiation has been used to modify the surface of ePTFE. See, Yotoriyama et al., Ion-Beam Irradiated ePTFE For The Therapy Of Intracranial Aneurysms”, No Shinkei Geka 2004, 32, pages 471-478; Takahashi et al, “Biocompatibility Of ePTFE Modified By Ion Beam Irradiation”, No Shinkei Geka 2004, 32, pages 339-344. Yotoriyama and Takahashi report improved cell adhesion to the irradiated ePTFE surfaces. The surfaces were irradiated with argon, helium, krypton and neon ions by ion beam techniques with high ion energy levels of 150 keV. Such high energy levels, however, were reported in Yotoriyama to destroy the fibrils of the ePTFE.
Thus, there is a need in the art for modifying the surface of ePTFE to promote cell adhesion without the disadvantages of the prior art. In particular, there is the need for modifying the surface of ePTFE to promote cell adhesion without destroying the node and fibril structure of the ePTFE.